pgsql: Change pg_restore -f- to dump to stdout instead of to ./-

Change pg_restore -f- to dump to stdout instead of to ./-

Starting with PostgreSQL 12, pg_restore refuses to run when neither -d
nor -f are specified (c.f. commit 413ccaa74d9a), and it also makes "-f -"
mean the old implicit behavior of dumping to stdout. However, older
branches write to a file called ./- when invoked like that, making it
impossible to write pg_restore scripts that work across versions. This
is a partial backpatch of the aforementioned commit to all older
supported branches, providing an upgrade path.

Discussion: https://postgr.es/m/20191006190839.GE18030@telsasoft.com

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d38635725cad3fb0a980e02440f5691c9ba00f3f

Modified Files
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doc/src/sgml/ref/pg_restore.sgml | 4 ++--
src/bin/pg_dump/pg_backup_archiver.c | 7 ++++++-
src/bin/pg_dump/pg_restore.c | 2 +-
3 files changed, 9 insertions(+), 4 deletions(-)